Internal combustion engines, particularly those of the reciprocating piston type, currently propel most vehicles. Such engines are relatively efficient, compact, lightweight, and inexpensive mechanisms by which to convert highly concentrated energy in the form of fuel into useful mechanical power.
Typically, a vehicle is propelled by such an engine, which is started from a cold state by a small electric motor and relatively small electric storage batteries, then quickly placed under the loads from propulsion and accessory equipment. Such an engine is also operated through a wide range of speeds and a wide range of loads and typically at an average of approximately a fifth of its maximum power output.
A vehicle transmission typically delivers mechanical power from an engine to the remainder of a drive system, such as fixed final drive gearing, axles and wheels. A typical mechanical transmission allows some freedom in engine operation, usually through alternate selection of five or six different drive ratios, a neutral selection that allows the engine to operate accessories with the vehicle stationary, and clutches or a torque converter for smooth transitions between driving ratios and to start the vehicle from rest with the engine turning. Transmission gear selection typically allows power from the engine to be delivered to the rest of the drive system with a ratio of torque multiplication and speed reduction, with a ratio of torque reduction and speed multiplication known as overdrive, or with a reverse ratio.
To operate properly, the transmission usually requires a supply of pressurized fluid, such as conventional transmission oil. The pressurized fluid may be used for such functions as cooling, lubrication, and, in some cases, operation of the torque transfer devices. The lubricating and cooling capabilities of transmission oil systems impact the reliability and durability of the transmission. Additionally, multi-speed transmissions require pressurized fluid for controlled engagement and disengagement of the torque transmitting mechanisms that operate to establish the speed ratios within the internal gear arrangement.
In hybrid vehicles, alternative power is available to propel the vehicle, minimizing reliance on the engine for power, thereby increasing fuel economy. Since hybrid vehicles can derive their power from sources other than the engine, engines in hybrid vehicles can be turned off while the vehicle is propelled by the alternative power source(s). For example, electrically variable transmissions alternatively rely on electric motors housed in the transmission to power the vehicle's driveline.
An electric generator can transform mechanical power from the engine into electrical power, and an electric motor can transform that electric power back into mechanical power at different torques and speeds for the remainder of the vehicle drive system. These functions may be combined into a single electric machine, a motor/generator. An electric storage battery used as a source of power for propulsion may also be used, allowing storage of electrical power created by the generator, which may then be directed to the electric motor for propulsion or used to power accessory equipment.
A series hybrid system allows the engine to operate with some independence from the torque, speed and power required to propel a vehicle, so the engine may be controlled for improved emissions and efficiency. Such a system may also allow the electric machine attached to the engine to act as a motor to start the engine. This system may also allow the electric machine attached to the remainder of the drive train to act as a generator, recovering energy from slowing the vehicle and storing it in the battery by regenerative braking.
An electrically variable transmission in a vehicle can simply transmit mechanical power from an engine input to a final drive output. To do so, the electric power produced by one motor/generator balances the electrical losses and the electric power consumed by the other motor/generator. By using the above-referenced electrical storage battery, the electric power generated by one motor/generator can be greater than or less than the electric power consumed by the other. Electric power from the battery can allow both motor/generators to act as motors. Both motors can sometimes act as generators to recharge the battery, especially in regenerative vehicle braking.
A power-split transmission can use what is commonly understood to be “differential gearing” to achieve a continuously variable torque and speed ratio between input and output. An electrically variable transmission can use differential gearing to send a fraction of its transmitted power through a pair of electric motor/generators. The remainder of its power flows through another, parallel path that is mechanical.
One form of differential gearing, as is well known to those skilled in this art, may constitute a planetary gear set. However, it is possible to construct this invention without planetary gears, as by using bevel gears or other gears in an arrangement where the rotational speed of at least one element of a gear set is always a weighted average of speeds of two other elements.
A hybrid electric vehicle transmission system may include one or more electric energy storage devices. The typical device is a chemical electric storage battery, but capacitive or mechanical devices, such as an electrically driven flywheel, may also be included. Electric energy storage allows the mechanical output power from the transmission system to the vehicle to vary from the mechanical input power from the engine to the transmission system. The battery or other device also allows for engine starting with the transmission system and for regenerative vehicle braking.